Evaluation of 3D-Printed Solid Microneedles Coated with Electrosprayed Polymeric Nanoparticles for Simultaneous Delivery of Rivastigmine and N-Acetyl Cysteine

In the current study, coated microneedle arrays were fabricated by means of digital light processing (DLP) printing. Three different shapes were designed, printed, and coated with PLGA particles containing two different actives. Rivastigmine (RIV) and N-acetyl-cysteine (NAC) were coformulated via electrohydrodynamic atomization (EHDA), and they were incorporated into the PLGA particles. The two actives are administered as a combined therapy for Alzheimer's disease. The printed arrays were evaluated regarding their ability to penetrate skin and their mechanical properties. Optical microscopy and scanning electron microscopy (SEM) were employed to further characterize the microneedle structure. Confocal laser microscopy studies were conducted to construct 3D imaging of the coating and to simulate the diffusion of the particles through artificial skin samples. Permeation studies were performed to investigate the transport of the drugs across human skin ex vivo. Subsequently, a series of tape strippings were performed in an attempt to examine the deposition of the APIs on and within the skin. Light microscopy and histological studies revealed no drastic effects on the membrane integrity of the stratum corneum. Finally, the cytocompatibility of the microneedles and their precursors was evaluated by measuring cell viability (MTT assay and live/dead staining) and membrane damages followed by LDH release.

Continuous Removal of Single Cell Layers by Tape Stripping the Stratum Corneum - a Histological Study

Studies on the penetration of toxicologically or pharmaceutically relevant substances through the skin and, more specifically, through the stratum corneum (s.c.) often rely on the well-established method of tape stripping. Tape stripping involves the removal of skin layers by means of adhesive tape, which is usually followed by quantification of dermally applied substances in these layers. However, the amount of s.c. removed by each individual tape strip is still a matter of scientific debate. While some studies imply that the amount of s.c. adhering to each tape strip decreases with increasing depth into the s.c., others observed a constant removal rate. All these studies rely on the quantification of the amount of s.c. captured on individual or pooled tape strips. Here, we present an approach whereby we measured the amount of s.c. remaining on excised porcine skin in the process of tape stripping. Staining and bloating of the s.c. allowed to measure its thickness and to count individual s.c. layers, respectively. Histologically, we show that the s.c. remaining on the skin decreased linearly as a function of strips taken. We found that each tape strip removes about 0.4µm of s.c., which corresponds to approximately one cellular layer. With a high coefficient of determination (r²>0.95), we were able to linearly correlate the thickness of the remaining s.c., the number of remaining cell layers and the number of tape strips applied. Furthermore, we elaborate on possible reasons for the discrepancies reported in the scientific literature regarding the amount of s.c. removed by each tape strip.

Systematic investigation of factors, such as the impact of emulsifiers, which influence the measurement of skin barrier integrity by in-vitro trans-epidermal water loss (TEWL)

Trans-epidermal water loss (TEWL) has been the most widely used method to assess the integrity of the skin barrier and evaluate the irritation potential or the protective properties of topical products for many years. It detects the amount of water that diffuses across the stratum corneum (SC) to the external environment. As one of the most important functions of the skin is to keep water inside the body, an increase in TEWL is used to indicate the skin's impaired barrier function. So far, a variety of commercial instruments are available to measure the TEWL. Their applications mainly focus on the in-vivo TEWL measurements for dermatological examinations or formulation development. Recently, an in-vitro TEWL probe has also been commercially released enabling preliminary tests with excised skin samples. In our study, we first aimed to optimize the experimental procedures for detecting the in-vitro TEWL of porcine skin. Secondly, different kinds of emulsifiers were applied to the skin, including polyethylene glycol-containing emulsifiers (PEG-ylated emulsifiers), sorbitan esters, cholesterol, and lecithin. Sodium lauryl sulfate (SLS) was used as a positive control, and water as a negative control. Based on the findings, we established a protocol for accurately measuring the in-vitro TEWL values, emphasizing that the temperature of the skin sample should be constantly maintained at 32℃. Subsequently, the influences of emulsifiers on the in-vitro TEWL were analyzed. They indicated a significant skin barrier impairment of PEG-20 cetyl ether, PEG-20 stearyl ether, and SLS on in-vitro skin. Furthermore, we interestingly found that there consistently was an alteration of the TEWL values, even after the application of water to the skin. Our findings are of special interest, as the European Medicines Agency (EMA) recommends the use of in-vitro TEWL to determine skin barrier intactness during Franz cell experiments. Thus, this study provides a validated protocol for measuring the in-vitro TEWL and elucidates the impact of emulsifiers on the skin barrier. It also improves the understanding of tolerable variations of in-vitro TEWL and offers recommendations for its use in research.

In Vitro, Ex Vivo, and In Vivo Evaluation of Nanoparticle-Based Topical Formulation Against Candida albicans Infection

Ketoconazole is commonly used in the treatment of topical fungal infections. The therapy requires frequent application for several weeks. Systemic side effects, allergic reactions, and prolonged treatment are often associated with non-compliance and therapy failure. Hence, we developed an optimized topical antifungal gel that can prolong the release of drug, reduce systemic absorption, enhance its therapeutic effect, and improve patient compliance. Ketoconazole-loaded PLGA nanoparticles were prepared by the emulsion/solvent evaporation method and were characterized with respect to colloidal properties, surface morphology, and drug entrapment efficiency. The optimized ketoconazole-loaded PLGA nanoparticles and commercially available silver nanoparticles were incorporated into a Carbopol 934P-NF gel base. This arrangement was characterized and compared with commercially available 2% ketoconazole cream to assess physical characteristics of the gel, in vitro drug release, ex vivo skin permeation and retention, and in vivo studies on Wister male albino rats. The results showed that polymeric PLGA nanoparticles were very effective in extending the release of ketoconazole in our optimized formulation. Nanoparticles were smooth, spherical in shape, and below 200 nm in size which is consistent with the data obtained from light scattering and SEM images. The ex vivo data showed that our gel formulation could strongly reduce drug permeation through the skin, and more than 60% of the drug was retained on the upper surface of the skin in contrast to 38.42% of the commercial cream. The in vivo studies showed that gel formulation could effectively treat the infection. This study demonstrates that our topical gel could be effective in sustaining the release of drug and suggests its potential use as a possible strategy to combat antifungal-resistant Candida albicans.

Antibacterial PEGylated Solid Lipid Microparticles for Cosmeceutical Purpose: Formulation, Characterization, and Efficacy Evaluation

The development of efficacious means of delivering antioxidant polyphenols from natural sources for the treatment of skin diseases is of great interest for many cosmetic and pharmaceutical companies. Resveratrol (RSV) and Limonene (LIM) have been shown to possess good anti-inflammatory and antibacterial properties against Staphylococcus aureus infections responsible for many skin disorders, such as acne vulgaris. In this study, solid lipid microparticles are designed as composite vehicles capable of encapsulating a high amount of trans-RSV and enhancing its absorption through the stratum corneum. A microparticulate system based on mixture of PEGylate lipids, long-chain alcohols and LIM is able to entrap RSV in an amorphous state, increasing its half-life and avoiding inactivation due to isomerization phenomena, which represents the main drawback in topical formulations. Particles have been characterized in term of shape, size distribution and drug loading. Antimicrobial tests against S. aureus have highlighted that empty microspheres possess per se antimicrobial activity, which is enhanced by the presence of LIM, demonstrating that they can represent an interesting bactericide vehicle for RSV administration on the skin.

Biosynthesis of size-controlled gold nanoparticles using M. lucida leaf extract and their penetration studies on human skin for plastic surgery applications

Recently, majority of the studies were focusing on the nanoparticles (NPs) and their abilities of penetrating Stratum Corneum (SC), as they can be prominently utilized in the plastic surgeries. In the current work, we demonstrated the penetrating abilities of gold NPs (AuNPs) through anthropological skin with diameters of 10 and 15 nm, varying in sizes, with the help of Multiphoton Microscopy. In addition, we also demonstrated a rapid facile environment friendly process of synthesizing AuNPs of adjustable sizes with the help of aqueous M. lucida leaf extract. Surface plasmon resonance was performed to confirm the synthesis of AuNPs at 530 nm with the help of UV-vis spectrophotometer. By differentiating the quantities of M. lucida leaf aqueous extracts, we studied the reduction time, morphological differences and size of the AuNPs. By performing Fourier Transformation Infrared Spectroscopy (FTIR), UV-vis spectroscopy, Transmission Electron Microscopy (TEM), Powder X-ray Diffraction (XRD), Energy Dispersive X-ray Spectroscopy (EDAX) and Selected Area Electron Diffraction (SAED), we characterized the fabricated AuNPs. The further aggregation and growth of AuNPs was protected by the polyphenols in the oxidised form by having a coordination with the surface of AuNPs. Moreover, the experiments of skin penetration showed an effort to deeply examine the factors leading to the penetration of particles into the human skin. These responses indicate that NPs at the determined size ranges penetrate the SC in the same pattern of the drug molecules, mostly by the intercellular paths. These responses attained were essential for developing a unique transdermal transporter as well as for understanding the basic interaction of skin-NPs for the application of plastic surgeries.

Measurements of AMPs in stratum corneum of atopic dermatitis and healthy skin-tape stripping technique

Decreased levels of antimicrobial peptides (AMPs) in atopic dermatitis (AD) have previously been reported and have been linked to the increased susceptibility to skin infections found in AD patients. This study intents to identify AMPs: hBD-2, hBD-3, RNase7, psoriasin and LL-37 in AD patients and healthy controls, and determine concentrations in consecutive depths of the outer most skin layers. Tape stripping was used on lesional and non-lesional skin. From each skin site, 35 consecutive tape strips were collected and pooled in groups of 5. Commercially available ELISA kits were used to determine AMP concentration in stratum corneum samples. hBD-2, hBD-3, RNase7 and psoriasin were identified in stratum corneum samples. hBD-3-level was markedly higher in AD non-lesional skin compared to healthy controls, and a similar trend was observed for RNase7. Most AMPs were distributed evenly through 35 tape strips, implying a homogeneous distribution of antimicrobial defense in the outer most skin layers. The findings indicate that AD patients may not suffer from a general baseline deficiency in AMPs, and that the innate immune defense is present throughout the stratum corneum, both insights of importance for understanding the role of AMPs in AD.

Assessment of Raman spectroscopy as a fast and non-invasive method for total stratum corneum thickness determination of pig skin

Determination of total stratum corneum (SC) thickness is necessary to construct accurate SC drug concentration depth profiles that are used to evaluate the skin absorption of locally acting active components. Currently, different established methods such as the microscopic or gravimetric approach, estimation via transepidermal water loss or NIR densitometry are used. However, some of them represent time consuming strategies. In the present study, Raman spectroscopy was assessed as a non-invasive and fast method for total SC thickness estimation. All techniques employed in this study yielded comparable results with SC values of 11.15 ± 1.52 μm derived from Raman experiments, 10.22 ± 2.64 μm from NIR densitometry measurements and 10.91 ± 2.03 μm from light microscopy studies suggesting Raman spectroscopy as an appropriate and rapid method for total SC thickness determination. As a further objective of the study, the storage conditions of the skin samples during Raman measurements and the impact of keeping the skin on the cartilage during NIR densitometry measurements were investigated. Skin samples can be stored dry during Raman measurements, if immediate measurement is not feasible. Furthermore, skin samples for NIR densitometry studies should be kept on the cartilage during the stripping procedure to avoid SC thickness underestimation.

Quantitative detection of caffeine in human skin by confocal Raman spectroscopy--A systematic in vitro validation study

For rational development and evaluation of dermal drug delivery, the knowledge of rate and extent of substance penetration into the human skin is essential. However, current analytical procedures are destructive, labor intense and lack a defined spatial resolution. In this context, confocal Raman microscopy bares the potential to overcome current limitations in drug depth profiling. Confocal Raman microscopy already proved its suitability for the acquisition of qualitative penetration profiles, but a comprehensive investigation regarding its suitability for quantitative measurements inside the human skin is still missing. In this work, we present a systematic validation study to deploy confocal Raman microscopy for quantitative drug depth profiling in human skin. After we validated our Raman microscopic setup, we successfully established an experimental procedure that allows correlating the Raman signal of a model drug with its controlled concentration in human skin. To overcome current drawbacks in drug depth profiling, we evaluated different modes of peak correlation for quantitative Raman measurements and offer a suitable operating procedure for quantitative drug depth profiling in human skin. In conclusion, we successfully demonstrate the potential of confocal Raman microscopy for quantitative drug depth profiling in human skin as valuable alternative to destructive state-of-the-art techniques.

Characteristic differences in barrier and hygroscopic properties between normal and cosmetic dry skin. I. Enhanced barrier analysis with sequential tape-stripping

OBJECTIVE

Cosmetic dry skin often has a lower hydration level but a similar apparent barrier function, as measured by transepidermal water loss (TEWL), than that of the normal skin. To investigate the intrinsic difference in barrier property and moisture-holding ability between the cosmetic dry and normal skin, we developed a new clinical and data analysis procedure based on sequential tape-stripping with TEWL measurement, coupled with chemical analysis for protein and natural moisturizing factors (NMF) in the stratum corneum.

METHODS

A clinical study consisting of 64 healthy Caucasian female subjects with normal and cosmetic dry skin was conducted according to our clinical and data collection protocols. After the baseline visual dryness assessment, 20 tape-strips were placed and removed on each test site using D-Squame tapes. TEWL was measured at baseline and after the 5th, 10th, 15th and 20th tape-strips. All tapes were analysed for protein mass via chemical extraction and the Pierce BCA protein assay, as well as using an infrared densitometry device SquameScan 850A. The stratum corneum thickness and barrier quality (water transport resistance per thickness of the stratum corneum) were decoupled from the apparent barrier function using the TEWL and protein data.

RESULTS

A linear relationship between 1/TEWL and cumulative protein removal was observed for both normal and cosmetic dry skin. However, the slope of the linear relation was significantly steeper for normal skin, and significantly more protein was removed from cosmetic dry skin. The results showed that on average, the barrier quality of the stratum corneum of the normal skin is about 40% higher than that of the dry skin, whereas the stratum corneum of the dry skin is about 30% thicker than that of the normal skin. In addition, the amount of SC removal in sequential tape-stripping is generally non-uniform.

CONCLUSION

Our results demonstrated that there are characteristic differences in the barrier property between normal and cosmetic dry skin. In comparison to the normal skin, the stratum corneum of the cosmetic dry skin is considerably thicker, however, with a lower barrier quality. The results also showed that the amount of the SC removal in sequential tape-stripping is generally non-uniform. Therefore, the number of tape strips is not a good indicator for the tape-stripping depth.

Finite and infinite dosing: difficulties in measurements, evaluations and predictions

Due to the increased demand for reliable data regarding penetration into and permeation across human skin, assessment of the absorption of xenobiotics has been gaining in importance steadily. In vitro experiments allow for determining these data faster and more easily than in vivo experiments. However, the experiments described in literature and the subsequent evaluation procedures differ considerably. Here we will give an overview on typical finite and infinite dose experiments performed in fundamental research and on the evaluation of the data. We will point out possible difficulties that may arise and give a short overview on attempts at predicting skin absorption in vitro and in vivo.

Assessment of near-infrared densitometry for in situ determination of the total stratum corneum thickness on pig skin: influence of storage time

To perform accurate tape-stripping measurements and to control for site-specific and interindividual differences the amount of stratum corneum (SC) removed by each tape and the total SC thickness must be known. The purpose of this study was to evaluate the use of near-infrared (NIR) densitometry at λ = 850 nm for in situ determination of the total SC thickness. Quantitative tape stripping was performed on pig ear skin. The amount of SC removed by each tape was measured by NIR densitometry and by microprotein assay. Derived from the linear correlation between both measurements, a conversion factor was calculated that relates the individual NIR densitometry readings to the thickness of the SC on the corresponding tape (l(SC-tape) [µm] = (abs.(850) - abs.(850)(blank))/23.9). The total SC thickness was determined based on the accumulated values of all tapes applied in quantitative tape stripping and compared to the values obtained from microscopic cross sections of biopsies. The total SC thickness was correctly determined by infrared densitometry independent of storage time and conditions (4°C up to 24 h; -21°C up to 3 months) in comparison with the standard histological evaluation.

Mechanism and determinants of nanoparticle penetration through human skin

The ability of nanoparticles to penetrate the stratum corneum was the focus of several studies. Yet, there are controversial issues available for particle penetration due to different experimental setups. Meanwhile, there is little known about the mechanism and determinants of their penetration. In this paper the penetration of four model gold nanoparticles of diameter 6 and 15 nm, differing in surface polarity and the nature of the vehicle, through human skin was studied using multiphoton microscopy. This is in an attempt to profoundly investigate the parameters governing particle penetration through human skin. Our results imply that nanoparticles at this size range permeate the stratum corneum in a similar manner to drug molecules, mainly through the intercellular pathways. However, due to their particulate nature, permeation is also dependent on the complex microstructure of the stratum corneum with its tortuous aqueous and lipidic channels, as shown from our experiments performed using skin of different grades of barrier integrity. The vehicle (toluene-versus-water) had a minimal effect on skin penetration of gold nanoparticles. Other considerations in setting up a penetration experiment for nanoparticles were also studied. The results obtained are important for designing a new transdermal carrier and for a basic understanding of skin-nanoparticle interaction.

In vitro vs. in vivo tape stripping: validation of the porcine ear model and penetration assessment of novel sucrose stearate emulsions

Porcine ear skin is frequently used as a substitute for human skin in dermatological research and is especially useful for tape stripping experiments where the penetration of active substances into the uppermost skin layers is investigated. However, certain differences between the surface properties of these skin types exist, and reports on the comparability of tape stripping data obtained in vitro using porcine ear skin and data obtained in vivo on human forearm skin are scarce. Thus, we performed comparative tape stripping experiments in which the skin penetration of curcumin and fluorescein sodium from conventional microemulsions and hydrogels was investigated. In this context, the skin penetration potential of novel semi-solid macroemulsions and fluid nanoemulsions based on sucrose stearate was evaluated as well. The removed corneocytes were quantified by NIR-densitometry using recent correlation data for human and porcine proteins. The trends observed for the skin penetration into porcine ear skin were highly representative for the in vivo situation on human skin, confirming that the porcine ear is an excellent in vitro model for tape stripping experiments. Moreover, the validity of the NIR-densitometric approach for the quantification of both human and porcine stratum corneum proteins was confirmed in this study for the first time.

Semi-solid Sucrose Stearate-Based Emulsions as Dermal Drug Delivery Systems

Mild non-ionic sucrose ester surfactants can be employed to produce lipid-based drug delivery systems for dermal application. Moreover, sucrose esters of intermediate lipophilicity such as sucrose stearate S-970 possess a peculiar rheological behavior which can be employed to create highly viscous semi-solid formulations without any further additives. Interestingly, it was possible to develop both viscous macroemulsions and fluid nanoemulsions with the same chemical composition merely by slight alteration of the production process. Optical light microscopy and cryo transmission electron microscopy (TEM) revealed that the sucrose ester led to the formation of an astonishing hydrophilic network at a concentration of only 5% w/w in the macroemulsion system. A small number of more finely structured aggregates composed of surplus surfactant were likewise detected in the nanoemulsions. These discoveries offer interesting possibilities to adapt the low viscosity of fluid O/W nanoemulsions for a more convenient application. Moreover, a simple and rapid production method for skin-friendly creamy O/W emulsions with excellent visual long-term stability is presented. It could be shown by franz-cell diffusion studies and in vitro tape stripping that the microviscosity within the semi-solid formulations was apparently not influenced by their increased macroviscosity: the release of three model drugs was not impaired by the complex network-like internal structure of the macroemulsions. These results indicate that the developed semi-solid emulsions with advantageous application properties are highly suitable for the unhindered delivery of lipophilic drugs despite their comparatively large particle size and high viscosity.

In vitro human skin segmentation and drug concentration-skin depth profiles

Highly optimized methods for skin segmentation are provided using tape stripping in combination with infrared absorption measurements for stratum corneum (SC) and cryosectioning for deeper skin layers. Furthermore, an example is calculated for demonstration of the respective procedures.